1. Field of the Invention
The present invention relates to a method for treating at least one metallic surface for protection against thermally induced discolorations and to a covering, treated by means of the method according to the invention, for an exhaust system.
2. Description of the Related Art
Exhaust systems or parts, particularly metallic parts, that contact an exhaust system may discolor due to high temperatures in the respective exhaust system, and, for example, rainbow-like patterns may form.
Complicated work, such as, rubbing down, polishing and painting is necessary to eliminate this effect and to restore an original coloring. Unfortunately, such work does not prevent the effect from recurring, and therefore renewed discoloration in the event of renewed heating has to be expected.
Against this background, a method for protecting at least one metallic surface against discolorations is presented, in which the at least one metallic surface is treated with a suitable clear lacquer. Clear lacquers of this kind are already known from the prior art and are offered, for example, under the trade name of NONA-X clear lacquer. After the method has been carried out, the clear lacquer forms a permanently effective oxygen barrier between the metallic surface and the hot exhaust gases of the exhaust system that flow over the metallic surface. Thus, the visual appearance of the metallic surface is not impaired. The oxygen barrier is composed of nanoparticles that form an oxygen-tight envelope that keeps oxygen away from the metal surface lying underneath and the discoloration of the metallic surface at high temperatures is prevented.
What has not yet been solved to date is that the metallic surface is damaged and discolored at the high temperatures prevailing in the stoving furnace.
It is proposed, in this regard, to seal the metallic surface against oxygen contact. This sealing may take place in various ways.
Thus, stoving may be carried out under a protective atmosphere. The term “under a protective atmosphere” is to be understood in the context of the present invention to mean treatment of the metallic parts in a closed space, for example in a stoving furnace, in which specific conditions are generated by means of selected gases. In particular, a protective atmosphere composed of nitrogen is capable of being used for the purpose of the displacement of oxygen.
Alternatively, there may be provision for subjecting the metallic surface to what is known as an INOX spectral method before the application and stoving of the clear lacquer. In this known electrochemical process, the metallic surface acquires coloring caused by the build-up of a chromium oxide layer. It was shown, surprisingly, that this effect hardens the metallic surface against oxygen contact in such a way that the stoving of the clear lacquer no longer has to take place under a protective atmosphere.
For both variants outlined above, it is recommended to carry out pretreatment of the metallic surface, for example by blasting with blasting stock and subsequent electropolishing. The blasting stock can in this case have a granulation of, in particular, 90 to 150 μm.